CO₂ Injection Pumps: Why the Clearance Volume Decides the Efficiency

Carbon dioxide has one point at which it stops behaving like a gas: 31.0 °C and 7.38 MPa. Above both, it is dense enough that a positive-displacement pump can move it, and at the Jilin oilfield that is the route the operator took — dense-phase injection replacing supercritical compressor injection. The machine developed for it is a plunger pump specified at a theoretical displacement of 15 m³/h, a suction pressure of 8.5–12 MPa, a maximum discharge pressure of 25 MPa and a medium temperature of 5–25 °C, with a stated pump efficiency of no less than 45%. That last figure is the project's own, and the project's documentation states plainly that dense-phase CO₂ injection pumps run at low efficiency both in China and abroad.

This article sets out why the duty is a pumping problem rather than a compression problem, what the machine looks like, why the clearance volume carries the efficiency, and where the published evidence stops.

Why is dense-phase CO₂ a pump duty rather than a compressor duty?

Because above the critical point the fluid's density is close to that of a liquid, and a machine that displaces volume handles it with far less work than a machine that compresses gas.

The operator's operating model on this project is stated as low-pressure supercritical transport with high-pressure supercritical injection, and the project documentation records that dense-phase injection replaced supercritical compressor injection. Two reasons are given for the change: dense-phase machines retain an advantage over compressors on input and output for this duty even at low efficiency, and developing a dense-phase pump specifically for the duty was therefore considered necessary.

The documentation also states the property that makes this fluid awkward regardless of which machine is chosen: its density varies sharply with pressure and temperature. A fluid whose density is a steep function of two variables does not behave like a liquid with a fixed density, and every volumetric machine loses efficiency when the volume it displaces and the mass it delivers stop being proportional.

What does the machine look like?

It is a plunger pump: a crankshaft converts the motor's rotation into reciprocating plunger motion, the plunger changes the volume of the plunger chamber, and check valves confine the medium to a single direction and raise its pressure.

Three developments are named in the published record.

A straight-through small-clearance quick-change valve. This is the central one, and its stated purpose is to reduce the valve clearance volume. The mechanism is described in the documentation: a smaller residual volume means less medium left in the chamber, which reduces the drop in density that the medium undergoes as its temperature rises during the suction stroke, which increases the mass drawn in and raises the efficiency.

A thermal-hydraulic coupled numerical model. The project states that it built a method for modelling the dynamic characteristics of a dense-phase CO₂ plunger pump using coupled thermal and flow simulation, to provide a modelling approach and reference data for optimising the design.

An injection-system design with automatic control. The system is stated to record and analyse its own parameters and to control the pressurisation and cooling systems automatically, holding the pump's inlet pressure and temperature within a set band so that the injection system works in its efficient region.

What do the published parameters say?

They are unusually complete for a machine at this stage, and they are worth reading as a set because the efficiency figure only makes sense next to the pressures.

The suction pressure is the number to look at first. A machine drawing at 8.5–12 MPa is already inside the supercritical region for carbon dioxide at the low end of its stated temperature band, and the compression ratio the plunger has to produce is modest — 25 MPa from about 10 MPa. What that means is that the work of compression is not the problem. The problem is holding on to the mass, and that is why the clearance volume, not the pressure ratio, is the design driver.

Why is 45% the published efficiency?

Because on this duty the efficiency is dominated by the fluid's behaviour inside the valve chamber rather than by the machine's mechanical quality, and the project states the figure honestly.

The documentation records that high-density CO₂ injection pumps currently have low efficiency generally, both in China and abroad, and it treats that as the starting condition of the development rather than as a defect of the machine it produced. The innovation list is aimed squarely at that number: the small-clearance valve, the coupled thermal and flow model, and the automatic control of inlet conditions are all ways of raising the mass delivered per stroke.

That is the honest form of the claim. A 45% figure on a duty like this is not a comparison with a water pump; it is a statement that for a fluid whose density moves with temperature and pressure, a volumetric machine loses a large share of its displacement to the residual volume and to the change of state inside the machine.

What does the rest of the equipment set look like?

The injection pump is one item in a chain, and the same supplier's public record describes the chain around it.

The company states that it has supplied the Jilin oilfield CCUS project with equipment from supercritical CO₂ line pipe through to wellhead injection equipment, and that the dense-phase injection pump's entry into batch application completes that set. The line pipe is described as having the largest wall thickness and largest diameter in China, and as capable of transporting CO₂ at ten-million-tonne scale. The wellhead injection and production equipment is described as using a multi-seal structure and corrosion-resistant, long-life materials.

Read as a technology statement rather than as a product list, the set says something about where the difficulty sits in carbon-capture injection. The transport side is a strength-and-toughness problem solved in steel — wall thickness and diameter. The wellhead side is a sealing problem. And the injection side is the efficiency problem this article is about.

Where does this route stop?

At four points, all of them stated or implied by the sources.

45% is a design figure, and no in-service efficiency is published. The number comes from the project documentation. No measured efficiency from operating machines, no operating hours, no mean time between failures and no overhaul interval appear in the record.

The route is depth-specific. It was chosen for an oilfield whose reservoir plan called for low-pressure supercritical transport and high-pressure supercritical injection. The transport side of the same project is supercritical line pipe, which is a different pressure and temperature band from the injection side. Treating "CCUS CO₂ equipment" as one specification would blur two distinctly different duties.

The architecture is one answer, not the answer. The published machine is a plunger pump with a small-clearance valve. The documentation's own reason for choosing a pump — that a pump retains an advantage over a compressor on input and output despite low efficiency — is a statement about this duty's economics, not a general rule that pumps displace compressors above the critical pressure.

The equipment set is described by one supplier. The line pipe, the wellhead equipment and the injection pump are all reported by the company that supplies them. Each item is described, and none is independently benchmarked in the public record.

What does the trend add up to?

Three movements are visible. Injection moved from compressors to plunger pumps at a specific pressure and temperature band, which is a change of machine class rather than a refinement. The design work concentrated on the valve chamber rather than on the plunger or the drive, because that is where the mass is lost. And the applications moved from a single research project to batch deployment on a live field, which is the step that turns a design record into an equipment class.

What is missing is the one thing that would settle the question: a published in-service efficiency for these machines.

What this article does not cover

Note that this article does not select a pump, size a plunger or a drive, calculate efficiency, specify a valve or a material, or give prices and delivery times. It describes the phase behaviour that makes the duty what it is, the machine and its named developments, the published parameters and the boundary of that evidence. It carries no compliance determination.

Where the next constraint sits

The clearance-volume problem in this article is a mass-retention problem, and the same class of problem appears on a duty where the fluid's temperature window, not its density, is the deciding fact: Molten-Salt Pumps: Why Temperature Decides the Machine, Not Pressure sets out what happens when the fluid is only liquid because it is kept hot, and the machine has to be able to empty itself before it can be stopped.

The temperature end of the same family — a fluid that is one heat leak away from boiling, rather than one that changes density with pressure — is worked in Liquid Hydrogen Pumps: What the −253 °C Duty Demands.

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